Field of the Invention
The present disclosure relates generally to methods of using a etch solutions with particular surface active agents to reduce the tendency for overetching and surface erosion during etching of high purity cubicity anode foil. The disclosure also relates to electrolyte bath compositions for such use, to etched foils produced by such methods, and to electrolytic capacitors.
Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable cardioverter defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size and ability to withstand relatively high voltage. Conventionally, an electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. The electrolyte impregnated in the separator functions as the cathode in continuity with the cathode foil, while an oxide layer on the anode foil functions as the dielectric.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, increasing the surface area of the aluminum anode foil results in increased capacitance per unit volume of the electrolytic capacitor. By electrolytically etching aluminum foils, enlargement of the foil surface area occurs. As a result of this enlarged surface area, electrolytic capacitors, manufactured with these etched foils, can obtain a given capacity with a smaller volume than an electrolytic capacitor which utilizes a foil with an unetched surface.
In a conventional electrolytic etching process, foil surface area is increased by removing portions of the aluminum foil to create etch tunnels. While electrolytic capacitors having anodes and cathodes comprised of aluminum foil are most common, anode and cathode foils of other conventional valve metals such as titanium, tantalum, magnesium, niobium, zirconium and zinc are also used. Electrolytic etching processes are illustrated in U.S. Pat. Nos. 4,213,835, 4,420,367, 4,474,657, 4,518,471, 4,525,249, 4,427,506, and 5,901,032.
In conventional processes for etching aluminum foil, an electrolytic bath is used that contains a sulfate, a halide, and an oxidizing agent, such as sodium perchlorate, such as the processes disclosed in U.S. Pat. Nos. 8,871,358, 8,038,866, 7,578,924, 6,858,126, and 6,238,810. Aluminum electrolytic capacitors' energy density is directly related to the surface area of the anodes generated in the electrochemical etching processes. Typical surface area increases are 40 to 1 and represent 30 to 40 million tunnels/cm2. An electrochemical widening step is used to increase the tunnel diameter after etching to insure the formation oxide will not close off the tunnels. Closing off of the tunnels during formation will reduce capacitance and electrical porosity.
It would be advantageous to utilize an etch process, particularly for a direct current (DC) etch process, using agents that prevent closing off of the tunnels during formation and increase foil capacitance and anode strength.